In a conventional belt type continuously variable transmission, a belt is wound around a primary pulley and a secondary pulley, and a shift operation is performed by varying respective pulley widths of the primary pulley and secondary pulley. The pulley width of each pulley varies according to a clamping force of each pulley, and a required clamping force is calculated on the basis of an input torque, a belt winding radius, and so on. In a conventional technique for preventing belt slippage, a predetermined safety factor having a fixed value is multiplied by the clamping force to set the clamping force at a value having a margin relative to the input torque. JP10-30698A discloses a technique for preventing Low side belt slippage by setting the safety factor at a steadily larger value as a shift value moves toward a Low side, rather than at a fixed value.